When a magnetic recording medium is used, the medium is rotated and accelerated rapidly from a stationary state, whereby a lift is given to a head slider and a magnetic head is lifted. When the power is switched off after the use, a motor driving the medium for rotation will stop, and the head and the medium will come into contact with each other at a high speed and cause sliding.
A thin layer magnetic recording medium, which is a typical example of the magnetic recording medium providing a high recording density used in the information industry and the like, is usually prepared by coating a magnetic metal or its alloy on a non-magnetic substrate by plating, vapor deposition, or sputtering. In actual use, it becomes abraded and damaged by the sliding contact of the magnetic recording medium with the head at a high speed. Its magnetic properties may be deteriorated.
As a method for solving such a problem, it has been proposed to provide a protective layer and a lubricant layer on the magnetic layer in order to make a static and dynamic friction coefficient of the medium during the sliding contact as low as possible and to improve an abrasion resistance. A carbonaceous layer, an oxide layer, a nitride layer, or a boride layer is employed as the protective layer. A liquid lubricant or a solid lubricant is employed as the lubricant layer. Generally, a perfluoropolyether compound, which is one of the liquid lubricants, is coated on the surface of the medium.
In recent years, to obtain a higher recording density, it has become increasingly necessary to reduce the flying height of the head and to speed up the rotation of the medium. Thus, the substrate for medium has come to be smoothed. The provision of the liquid lubricant layer is very effective in reducing the dynamic friction coefficient, as mentioned in the above. As the thickness of the liquid lubricant layer increases, however, it has been found that a micromeniscus is formed by the surface tension of the liquid lubricant between the head and the medium and thereby causes sticking. Hence, this indicates that the static friction coefficient of the medium increases and the head frequently becomes inoperative due to its adhesion to the medium.
As the substrate is smoothed in order to reduce the flying height of the head, the liquid lubricant has a serious drawback in that the above-mentioned sticking is very liable to occur. On the other hand, it is disadvantageous in that a sufficient durability cannot be obtained when the thickness of the liquid lubricant layer is reduced to prevent the sticking. Further, as the rotation speed of the medium increases, there occurs a marked phenomenon called "spin-off" and the thickness of the lubricant layer is reduced. To avoid such phenomena, the search has gone on for a solid lubricant which does not form the meniscus and it has been proposed to use a higher alcohol, a higher fatty acid, or metal salts thereof.
The solid lubricant has a problem, however, in that, when it is coated on the magnetic layer, a crystallization is liable to take place in part of the coated layer which causes a cohesion, since its stable phase is as a solid at the ambient temperature. In particular, the tendency toward cohesion is marked when the substrate is smoothed. When the cohesion occurs, the thickness of the coated layer becomes uneven, which increases the possibility that the head will be brought into direct contact with the magnetic layer of the medium. In addition, a stain may form on the head slider or the flying height of the head may be made unstable. In order to prevent the cohesion of the solid lubricant and to prevent the spin-off, it is necessary to effectively bind and cause the molecules of the solid lubricant to adhere to the substrate. One approach that has been proposed to increase the bonding strength of the molecules of the lubricant has been a method of polymerizing an alkylsilane (Japanese Unexamined Patent Publications No. 103721/1990 and No. 103722/1990). However, according to this method, the lubricant is polymerized, whereby the movement of their molecules is restricted and the lubricating performance becomes inadequate. This is because there is a trade-off to be made between the fixation of the lubricant and its lubricating performance.
Further, even though the solid lubricant exerts a good lubricating performance in the initial stage, the friction coefficient markedly increases with the passage of time and no satisfactory lubricating performance can be obtained. The higher fatty acid, which has the effect of fairly maintaining the friction coefficient at a low level, has the disadvantage of becoming easily liquefied under the conditions in which it is brought into contact with the head at 50.degree. C. or higher because it generally has a low melting point and therefore has a tendency to cause the sticking of the head. On the one hand, to avoid such a disadvantage, it has been proposed to use the higher fatty acid in the form of a metal salt which has a higher melting point (e.g., Japanese Unexamined Patent Publication No. 281220/1988). The metal salt of higher fatty acid has a good lubricating performance. On the other hand, it has a drawback in that its solubility in a common organic solvent is poor and thus its coating possibilities are limited. In addition, there are problems in that an even coated layer is hardly obtained when the metal salt is coated on the medium and the thickness of the coated layer is considerably uneven. This tendency toward unevenness is particularly marked when the substrate is smoothed, which may possibly lead to serious results such as a head crash.
A reversible adhesion is ideal as a fixation system in which the lubricant molecules interact with the substrate while retaining the strength enough to prevent the cohesion and the adhesion does not use a trade-off with the lubricating performance.
An object of the present invention is to obtain a lubricant layer which is a thin layer but which has an excellent lubricating performance and is free from the cohesion of the lubricant molecules, that is, a lubricant layer excellent in durability without introducing a trade-off between the adhesion of the lubricant and its lubricating performance. The present invention has been achieved by designing the lubricating system at a molecular or atomic level. The present invention also provides a magnetic recording medium having an even and thin lubricant layer which is excellent in lubricating performance and durability and which is formed by using the above lubricant layer.